A. Field of Invention
The present invention generally pertains to analog to digital converters, and more particularly pertains to quantizers.
B. Description of the Background
Communications systems frequently use high frequency carrier signals that are modulated by information for transmission from a transmitter to a receiver. On the receiver end, the modulated signal is demodulated to retrieve the information that has been transmitted over the transmission medium. The transmission medium may, for example, constitute free space, electrical wires such as coaxial cables and telephone wires, fiber optics, acoustical transmission media, etc. Various types of waves can be used to transmit these modulated signals, including electromagnetic waves, magnetic waves, acoustical waves, etc.
Significant noise interference may occur as a result of the transmission medium. For example, atmospheric interference may cause noise of various types in the transmission of electromagnetic waves over free space. In wire transmission systems, such as coaxial cable or twisted pair networks, active components may cause nonlinear effects that can result in significant noise and/or distortion of the transmitted signal. Additionally, in transmitting signals such as quadrature amplitude modulation signals (QAM), a slight error in the phase of the carriers at the demodulator will not only result in the possible loss of the signal, but may also lead to interference between the two channels. Similar difficulties arise when the local frequency is in error. In addition, unequal attenuation of the upper and lower sidebands during transmission also leads to cross-talk or co-channel interference. Other types of noise can also occur, such as attenuation distortion, envelope delay distortion, incidental phase disturbances, including phase jitter, phase wobble, and phase hits that cause the transmitted signal to experience acceleration, slowdown, or even sudden discontinuities. Other sources of noise and distortion may include impulse noise and background noise, thermal noise, harmonic distortion and intermodulation distortion.
The information that is transmitted by modulating a carrier may comprise digital signals, including multilevel digital signals. Difficulties frequently arise in the ability to correctly quantize an analog signal to retrieve the correct digital signal at the receiver. In order to correctly quantize a multilevel signal, decision thresholds must be established to distinguish between different levels of the digital signal.
A typical approach for establishing the appropriate level of a decision threshold in a multilevel signal is to set the decision threshold at a midway point between the levels of the transmitted signal level. This is based on the assumption that noise and other distortion will equally affect the signal regardless of the amplitude or phase of the signal. Assuming that the received signals are equally as likely to be affected by the noise or distortion, on average the smallest possible error will occur by placing the threshold halfway between the two levels of the transmitted signal.
However, when nonlinear distortions occur and nonlinear noise affects the received signal, the optimum level for setting the decision threshold does not fall halfway between the levels of the transmitted signal. For example, amplitude-dependent noise will cause a signal at a higher level to cross over the decision threshold that is placed halfway between two levels more often than a lower amplitude signal, which will not have as many excursions across the decision threshold. In other words, since the larger amplitude signal will be more greatly affected by noise, the larger amplitude signal will have more frequent occurrences when it crosses over the threshold than a lower amplitude signal because the noise is more severe at a higher amplitude state.
It is therefore desirable to provide quantizer which adaptively places the decision threshold at a level that minimizes the overall error rate.